Porphyrin dyes have attractive photophysical properties, which can be exploited in optochemical sensors for the detection of a number of important analytes [1]. Their chromophoric moiety consists of an aromatic tetrapyrolic macrocycle, which can accommodate central ligand(s) such as metal ions or protons, and peripheral substituents in pyrrol and meso-positions. Modifications can also involve the macrocycle itself, giving rise to chlorins, benzoporphyrins, porphyrin-ketones, aza-porphyrins with characteristic spectral properties [2]. These features provide flexibility in tuning the optical (absorption, luminescence) and physical-chemical (hydrophilicity, linkers, additional functionality) characteristics of porphyrin dyes and in designing new reporter molecules for sensing applications.
In particular, bright, long-decay phosphorescence of Pt(II)- and Pd(II) porphyrins have allowed the development of simple and robust lifetime-based oxygen (O2) sensors which are widely used nowadays and gaining popularity in many areas. Several other analytes including temperature, SO2, NOx and relative humidity have been analysed by phosphorescence quenching of metalloporphyrin dyes incorporated in solid-state materials [3]. Using more complex composites and combinations of dyes, it became possible to extend the range of analytes amenable to quenched-phosphorescence sensing and to achieve multi-parametric detection [4]. However, cross-sensitivity, compromised performance, increased complexity and manufacturing costs often limit the use of such sensors. There is therefore a need for more simple and robust multi-analyte sensor systems.
Besides O2, optochemical sensors for pH (as well as sensors for acidic and basic species such CO2, SO2, ammonia which can be constructed on the basis of a pH transducer [5-7]) are of particular interest. pH sensors comprising protonable porphyrin dyes embedded in a placticised PVC polymer which rely on the changes in absorption and fluorescence of the porphyrin dyes have been described [12]. However the use of phosphorescence modality dual-analyte sensing approaches have not been demonstrated with such sensor systems.